This application claims the benefit of Korean Patent Applications No. 2000-20723 filed on Apr. 19, 2000 and No. 2000-53614 filed on Sep. 8, 2000, which are hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment orientation of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed.
A liquid crystal is classified into a positive liquid crystal and a negative liquid crystal, in view of electrical property. The positive liquid crystal has a positive dielectric anisotropy such that long axes of liquid crystal molecules are aligned parallel to an electric field. Whereas, the negative liquid crystal has a negative dielectric anisotropy such that long axes of liquid crystal molecules are aligned perpendicular to an electric field.
By now, an active matrix LCD that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is most attention-getting due to its high resolution and superiority in displaying moving video data.
FIG. 1 is a cross-sectional view illustrating a typical twisted nematic (TN) LCD panel. As shown in FIG. 1, the TN-LCD panel has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2 includes a first transparent substrate 1a and a thin film transistor (xe2x80x9cTFTxe2x80x9d) xe2x80x9cSxe2x80x9d. The TFT xe2x80x9cSxe2x80x9d is used as a switching element to change orientation of the liquid crystal molecules. The lower substrate 2 further includes a pixel electrode 15 that applies an electric field to the liquid crystal layer 10 in accordance with signals applied by the TFT xe2x80x9cSxe2x80x9d. The upper substrate 4 has a second transparent substrate 1b, a color filter 8 on the second transparent substrate 4, and a common electrode 14 on the color filter 8. The color filter 8 implements color for the LCD panel. The common electrode 14 serves as another electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 15 is arranged over a pixel portion xe2x80x9cP,xe2x80x9d i.e., a display area. Further, to prevent leakage of the liquid crystal layer 10 between the lower and upper substrates 2 and 4, those substrates are sealed by a sealant 6.
As described above, because the pixel and common electrodes 15 and 14 of the conventional TN-LCD panel are positioned on the lower and upper substrates 2 and 4, respectively, the electric field induced therebetween is perpendicular to the lower and upper substrates 1a and 1b. The above-mentioned liquid crystal display device has advantages of high transmittance and aperture ratio, and further, since the common electrode on the upper substrate serves as an electrical ground, the liquid crystal is protected from a static electricity.
FIGS. 2A and 2B show different alignments of the positive TN liquid crystal molecules 10, respectively, without and with an electric field (off and on states). In FIG. 2A, various arrows show the gradual rotating of the liquid crystal molecules 10 with polar angles 0 to 90 degrees, which are measured on a plane parallel to the lower and upper substrate 2 and 4. At the same time, the liquid crystal molecules 10 are gradually rotated to 90 degrees from the lower substrate 2 to the upper substrate 4. That is to say, the long axes of the liquid crystal molecules 10 gradually rotate along a helical axis (not shown) that is perpendicular to the lower and upper substrates 2 and 4. First and second polarizers 18 and 30 are positioned on the exterior surfaces of the lower and upper substrate 2 and 4, respectively. At this point, the broken lines on the first and second polarizers 18 and 30 correspond to first and second transmittance axis of the first and second polarizers 18 and 30, respectively. After rays of light travel through a TN liquid crystal panel in the off state, as discussed above, they are linearly polarized and rotated 90 degrees.
As shown in FIG. 2B, when there is an electric field xe2x80x9cExe2x80x9d applied to the positive TN liquid crystal molecules 10, the liquid crystal molecules are aligned perpendicular to the upper and lower substrates 4 and 2. That is to say, with the electric field E applied across the liquid crystal molecules 10, the liquid crystal molecules 10 rotate to be parallel to the electric field xe2x80x9cExe2x80x9d. In this case, the rotation of the linearly polarized light does not take place. Therefore, light is blocked by the second polarizers 30 after it travels through the first polarizer 18.
However, the above-mentioned operation mode of the TN-LCD panel has a disadvantage of a narrow viewing angle. That is to say, the TN liquid crystal molecules rotate with polar angles 0 to 90 degrees, which are too wide. Because of the large rotating angle, contrast ratio and brightness of the TN-LCD panel fluctuate rapidly with respect to the viewing angles.
To overcome the above-mentioned problem, an in-plane switching (IPS) LCD panel was developed. The IPS-LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN or STN (super twisted nematic) LCD panel. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided with reference to FIGS. 3, 4A, and 4B.
As shown in FIG. 3, first and second substrates 1a and 1b are spaced apart from each other, and a liquid crystal xe2x80x9cLCxe2x80x9d is interposed therebetween. The first and second substrates 1a and 1b are called an array substrate and a color filter substrate, respectively. Pixel and common electrodes 15 and 14 are disposed on the first substrate 1a. The pixel and common electrodes 15 and 14 are parallel with and spaced apart from each other. On a surface of the second substrate 1b, a color filter 25 is disposed opposing the first substrate 1a. The pixel and common electrodes 15 and 14 apply an electric field xe2x80x9cExe2x80x9d to the liquid crystal xe2x80x9cLCxe2x80x9d. The liquid crystal xe2x80x9cLCxe2x80x9d has a negative dielectric anisotropy, and thus it is aligned parallel to the electric field xe2x80x9cExe2x80x9d.
FIGS. 4A and 4B conceptually illustrate operation modes for a typical IPS-LCD device. In an off state, the long axes of the LC molecules xe2x80x9cLCxe2x80x9d maintain a definite angle with respect to a line that is perpendicular to the pixel and common electrodes 15 and 14. The pixel and common electrode 15 and 14 are parallel with each other. Herein, the angle difference is 45 degrees, for example.
In an on state, an in-plane electric field xe2x80x9cExe2x80x9d, which is parallel with the surface of the first substrate 1a, is generated between the pixel and common electrodes 15 and 14. The reason is that the pixel electrode 15 and common electrode 14 are formed together on the first substrate 1a. Then, the LC molecules xe2x80x9cLCxe2x80x9d are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the LC molecules xe2x80x9cLCxe2x80x9d are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes 15 and 14.
In the above-mentioned IPS-LCD panel, there is no transparent electrode on the color filter, and the liquid crystal used in the IPS-LCD panel includes a negative dielectric anisotropy.
FIGS. 5A and 5B are conceptual plane views illustrating alignment of the liquid crystal molecules of the above-mentioned IPS-LCD panel, respectively, in off and on states. As shown in FIG. 5A, each liquid crystal molecule 10 is aligned in a proper direction by a pair of alignment layers (not shown), which are formed on opposing surfaces of the first and second substrate 1a and 1b. As shown in FIG. 5B, the electric field xe2x80x9cExe2x80x9d is applied between the pixel and common electrodes 15 and 14 such that each molecule 10 is aligned in accordance with the electric field xe2x80x9cExe2x80x9d. That is to say, each liquid crystal molecule 10 rotates to a definite angle in accordance with the electric field xe2x80x9cExe2x80x9d.
Compared with the TN-LCD device of FIG. 1, the IPS-LCD device has a wider viewing angle owing to a smaller rotating angle of the liquid crystal molecules.
The IPS-LCD device has the advantage of a wide viewing angle. Namely, when a user looks at the IPS-LCD device in a top view, the wide viewing angle of about 70 degrees is achieved in up, down, right and left directions.
By the above-mentioned operation modes and with additional elements such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has a wide viewing angle, low color dispersion qualities, and the fabricating processes thereof are simpler among those of various LCD devices.
However, because the pixel and common electrodes are disposed on the same plane on the lower substrate, the transmittance and aperture ratio are low. In addition, a response time according to a driving voltage should be improved, and a color""s dependence on the viewing angle should be decreased.
FIG. 6 is a graph of the CIE (Commission Internationale de l""Eclairage) color coordinates and shows the color dispersion property of the conventional IPS-LCD device. The horseshoe-shaped area is the distribution range of the wavelength of visible light. The results are measured using point (0.313, 0.329) in CIE coordinate as a standard white light source and with various viewing angles of right, left, up and down, and 45 and 135 degrees. Obviously, the range of the color dispersion is so long, which means that the white light emitted from the conventional IPS-LCD device is dispersed largely according to the viewing angle. This results from the fact that the operation mode of the IPS-LCD device is controlled by birefringence. S. Endow et al. indicated the above-mentioned problem in their paper xe2x80x9cAdvanced 18.1-inch Diagonal Super-TFT-LCDs with Mega Wide Viewing Angle and Fast Response Speed of 20 ms: IDW 99xe2x80x2 187 pagexe2x80x9d.
FIG. 7 is a graph illustrating transmittance with respect to viewing angles for first to eighth gray levels (gray scale) of a conventional IPS-LCD device. Except for the first gray level, xe2x80x9clevel 1,xe2x80x9d each gray level has the highest transmittance at a viewing angle of 0 degree. The first gray level, xe2x80x9clevel 1,xe2x80x9d has gray inversion regions. When the viewing angle is beyond 60 degrees, the first gray level, xe2x80x9clevel 1,xe2x80x9d has the higher transmittance than the fourth gray level, xe2x80x9clevel 4.xe2x80x9d The first gray level, xe2x80x9clevel 1,xe2x80x9d should implement a black state of the LCD panel. However, gray inversion occurs at viewing angles larger than 60 degrees, such that a white state, but not a black state, is produced at the larger viewing angles. The above-mentioned gray inversion results from a birefringence dependence of the IPS-LCD device and causes a poor display quality of the IPS-LCD device.
FIG. 8 shows an example of the IPS-LCD device according to the related art. As shown in FIG. 8, zigzag-shaped pixel electrodes 35 and zigzag-shaped common electrodes 34 are alternately arranged such that first and second electric fields 46a and 46b are alternately induced along the zigzag-shaped electrodes. The first and second electric fields 46a and 46b have different directions. Therefore, a multi-domain is achieved owing to the first and second electric fields 46a and 46b. An alignment layer (not shown) is also used for a first state alignment of liquid crystal molecules (reference 10 of FIG. 3). The alignment layer (not shown) beneficially has one rubbing direction 44.
The above-mentioned zigzag-shaped common and pixel electrodes 34 and 35 minimize the color shift. However, between bending portions xe2x80x9cDxe2x80x9d of the common and pixel electrodes 34 and 35, an electric field is induced perpendicular to the rubbing direction 44. That is to say, long axes of the liquid crystal molecules are perpendicular to the electric field induced between the bending portions xe2x80x9cD.xe2x80x9d Then, the liquid crystal molecules cannot rotate, but keep the first state alignment such that an abnormal alignment is present at each boundary portion xe2x80x9cCxe2x80x9d between the different domains.
The abnormal alignment at the boundary portion xe2x80x9cCxe2x80x9d causes a light leak such that white lines are shown on a display area, the pixel region xe2x80x9cPxe2x80x9d shown in FIG. 1, of the LCD device. The above-mentioned white lines are called a disclination. A black matrix may be expanded to the pixel regions to cover the disclination. However, the expanded black matrix causes a low aperture ratio.
Now, with reference to FIGS. 9A and 9B, effect of the multi-domain is explained in detail. A liquid crystal layer generally has a birefringence, because each liquid crystal molecule has a long and thin shape. The birefringence changes with respect to a viewing angle. FIG. 9A is a cross-sectional view illustrating a single-domain for a liquid crystal molecule 10 between upper and lower polarizers 30 and 18. At this point, the birefringence of the liquid crystal molecule 10 involves different values for the first, second, and third position xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, and xe2x80x9ccxe2x80x9d, which involve different viewing angles. Therefore, the birefringence of the liquid crystal molecule 10 cannot be zero with respect to viewing angles. If the birefringence of the liquid crystal layer is not zero, the perfect black state cannot be achieved between the upper and lower polarizers 30 and 18.
To overcome the above-mentioned problem, the multi-domain shown in FIG. 9B is adopted for a LCD device. As shown, there are first and second liquid crystal molecules 10a and 10b arranged opposite to each other. The birefringence of the first liquid crystal molecule 10a involves different values for the first, second, and third position xe2x80x9ca1xe2x80x9d, xe2x80x9cb1xe2x80x9d, and xe2x80x9cc1xe2x80x9d Whereas, the birefringence of the second liquid crystal molecule 10b involves different values for the fourth, fifth, and sixth position xe2x80x9ca2xe2x80x9d, xe2x80x9cb2xe2x80x9d, and xe2x80x9cc2.xe2x80x9d The first and fourth positions xe2x80x9ca1xe2x80x9d and xe2x80x9ca2xe2x80x9d involve the same viewing angle. Because the first and second liquid crystal molecules 10a and 10b are symmetrically opposed with each other, birefringence of the first liquid crystal molecule 10a at the first position xe2x80x9ca1xe2x80x9d is compensated by that of the second liquid crystal molecule 10b at the fourth position xe2x80x9ca2xe2x80x9d That is to say, each birefringence of the first liquid crystal molecule 10a is compensated by corresponding birefringence of the second liquid crystal molecule 10b. In other words, sum of the birefringence between the first and second liquid crystal molecules 10a and 10b is about zero. Accordingly, the multi-domain shown in FIG. 9B improves the display quality of the LCD device.
Accordingly, the present invention is directed to an IPS-LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an IPS-LCD device having low color dispersion and low white inversion with respect to viewing angles.
Another object of the present invention is to provide an IPS-LCD device having optimized common and pixel electrodes such that high aperture ratio, low color shift, and fast response time are achieved.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve the above object, the present invention provides an IPS-LCD device, which includes: first and second substrates opposing each other; a gate line on the first substrate; a data line perpendicular to the gate line; a thin film transistor at a crossing portion between the gate and data lines; a common line parallel to the gate line; a plurality of common electrodes electrically connected to the common line, wherein the common electrodes are spaced apart from each other; a plurality of pixel electrodes alternately arranged with the plurality of common electrodes, wherein each pixel electrode is spaced apart from an adjacent common electrode; a plurality of dielectric protrusions between the first and second substrates; and a liquid crystal layer between the first and second substrates, wherein the liquid crystal layer and the dielectric protrusion have different dielectric constants.
The dielectric protrusion has a smaller or larger dielectric constant than the liquid crystal layer.
The dielectric protrusion is an organic material, which is preferably selected from a group consisting of photoresist, benzocyclobutene (BCB), and acryl resin.
A plurality of first dielectric protrusions are disposed over a plurality of pixel electrodes. A plurality of second dielectric protrusions are disposed over a plurality of common electrodes. The plurality of first and second protrusions are formed on the first substrate having the pixel electrodes, or the plurality of first and second protrusions are formed on the second substrate.
The dielectric protrusion is a chevron-shaped dielectric protrusion, and the chevron-shaped dielectric protrusion has a zigzag shape extending along a line perpendicular to the common and pixel electrodes.
The pixel electrode is selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO). The common electrode is selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), antimony (Sb), and an alloy thereof. The common electrode is further selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO).
The liquid crystal layer is a positive liquid crystal having a positive dielectric anisotropy, and long axes of liquid crystal molecules are aligned parallel to the common and pixel electrodes in off state. In another aspect, the liquid crystal layer is a negative liquid crystal having a negative dielectric anisotropy, and long axes of liquid crystal molecules are aligned perpendicular to the common and pixel electrodes in an off state.
In another aspect, the present invention provides an in-plane-switching liquid crystal display panel, which includes: first and second substrates opposing each other; a gate line on the substrate; a data line perpendicular to the gate line; a thin film transistor at a crossing portion between the gate and data lines; a main common line parallel to the gate line; first and second auxiliary common lines perpendicular to the main common line, the first and second auxiliary common lines being parallel to and spaced apart from each other; a plurality of common electrodes electrically connected to the common line, wherein the common electrodes are spaced apart from each other; a plurality of pixel electrodes alternately arranged with the plurality of common electrodes, wherein each pixel electrode is spaced apart from an adjacent common electrode; a plurality of dielectric protrusions between the first and second substrates; and a liquid crystal layer between the first and second substrates.
The dielectric protrusion has a smaller or larger dielectric constant than the liquid crystal layer.
The dielectric protrusion is an organic material, which is preferably selected from a group consisting of photoresist, benzocyclobutene (BCB), and acryl resin.
A plurality of first dielectric protrusions are disposed over a plurality of pixel electrodes. A plurality of second dielectric protrusions are disposed over a plurality of common electrodes. The plurality of first and second protrusions are formed on the first substrate having the pixel electrodes, or the plurality of first and second protrusions are formed on the second substrate.
The dielectric protrusion is a chevron-shaped dielectric protrusion, and the chevron-shaped dielectric protrusion has a zigzag shape extending along a line perpendicular to the common and pixel electrodes.
The pixel electrode is selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO). The common electrode is selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), antimony (Sb), and an alloy thereof. The common electrode is further selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO).
The liquid crystal layer is a positive liquid crystal having a positive dielectric anisotropy, and long axes of liquid crystal molecules are aligned parallel to the common and pixel electrodes in off state. In another aspect, the liquid crystal layer is a negative liquid crystal having a negative dielectric anisotropy, and long axes of liquid crystal molecules are aligned perpendicular to the common and pixel electrodes in an off state.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.